The present invention relates to an anode-cathode structure for a sputter ion pump for use in ultra-high vacuum conditions.
FIG. 1 illustrates a conventional sputter ion pump in which a multi-cell anode comprises a number of hollow cylindrical members A which are parallel with each other and disposed between two cathode plates C. The cathode plates C are subjected to sputtering by means of penning discharging, to activate the surfaces thereof, and gas molecules are adsorbed or embedded in the activated surfaces of the cathode plates C, or gas molecules are caught by the surfaces of the anode, whereby evacuation of gases may be carried out.
In another conventional anode arrangement, polygonal hollow members may be used for a multi-cell type anode. In addition, a further anode structure is also known in which a plurality of plate members are layered on each other and are provided with a number of concentric holes, and the respective plate members are maintained at an equal potential.
A sputter ion pump currently used was developed and completed in the 1970s and its exhaust region of the pump was about 10.sup.-3 Pa to 10.sup.-9 Pa. When the pump was used to attain ultra-high vacuum conditions, the sputter ion pump was used in combination with a rotary vacuum pump or an absorption pump.
Thereafter, in the 1990s, a turbo molecule pump has been widely used. Primarily, an evacuation procedure is used in which the turbo molecule pump is firstly operated to perform a coarse or rough evacuation up to 10.sup.-5 Pa. Thereafter, a sputter ion pump is operated to attain an intended vacuum level. There have been demands for a sputter ion pump with an increased attainable critical vacuum level. Specifically, a sputter ion pump is able to perform evacuation up to 10.sup.-10 Pa, and having an evacuation speed maximized in the region of 10.sup.-7 Pa to 10.sup.-9 Pa has been desired.
As a method of increasing the obtainable critical vacuum level, a conventionally known method is used in which the product of the intensity (B) of a magnetic field and the diameter (D) of each hollow cylinder of an anode is increased to increase the ionization collision frequency of cathode emission electrons. (See Journal "Vacuum", Vol. 13, No. 7, p. 230.)
Meanwhile, J. Vac. Sci. Technol., Vol. 11, No. 6 teaches that the evacuation speed of a sputter ion pump is proportional to a length (L) of an anode and a diameter (D) of the respective hollow cylinder. In general, when the performance of magnets is kept constant, the intensity of a magnetic field can be increased by decreasing the distance between the magnets. In order to decrease the distance between the magnets, the length (L) of the anode must be shortened, and as a result, the evacuation speed of the pump is reduced. If the diameter (D) of the respective anode hollow cylinder is increased to attain a higher critical vacuum level, the number of hollow cylinders in a limited range of the magnetic field is decreased, and thus the evacuation speed of the pump is reduced. Also, if the space of the magnetic field is kept constant, it is impossible to extend the length (L) of the anode. Therefore, conventional sputter ion pumps sacrifice exhaust speed in order to increase the attainable critical vacuum level.
A report disclosed in J. Vac. Sci. Technol., Vol. 11, No. 6 says that the evacuation speed is proportional to the effective length (1+0.5 .delta.) of an anode. However, it has been found that this relation is not satisfied within a range of a low pressure or high vacuum. In particular, when the pressure is equal to or lower than 10.sup.-5 Pa, the evacuation speed is not proportional to the effective length (1+0.5 .delta.) of the anode.
Thus, a conventional sputter ion pump as shown in FIG. 1 suffers because evacuation speed must be sacrificed in order to increase the attainable critical vacuum level because the number of anode hollow cylinders existing in a limited range of a magnetic field is decreased. Thus, the evacuation speed of the pump is reduced when the diameter (D) of each anode hollow cylinder is increased to decrease the limit pressure. Moreover, the length (L) of the anode cannot be enlarged when the magnetic field is kept constant.
Furthermore, the conventional sputter ion pump as shown in FIG. 1 is designed by calculating the evacuation speed S.sub.1 of each anode cell (or discharging section) in accordance with following relation: EQU S.sub.1 .varies.Lr.sub.a.sup.2
where r.sub.a is the radius of the cell.
With a sputter ion pump having n discharging sections, therefore, the evacuation speed S.sub.n is represented as S.sub.n =nS.sub.1. However, the exhaust speed S.sub.o is actually lower than nS.sub.1 because of conductance in a gap between the anode and each of two cathodes.
Consequently, in order to increase the evacuation speed of the sputter ion pump, the length (L) of the anode and the gaps (G.sub.1) and (G.sub.2) between the cathodes and the anode must be increased. However, when the sum of (L)+(G.sub.1)+(G.sub.2) made larger, the intensity of the magnetic field is weakened as described above. Therefore, the conventional sputter ion pump is designed such that the length of the anode (L) is as large as possible, where that the sum of (L)+(G.sub.1)+(G.sub.2) is kept constant. That is, all of conventional sputter ion pumps are designed so as to satisfy a condition of (L)&gt;(G.sub.1)+(G.sub.2).